The invention relates to CATV systems, and more particularly, to CATV systems in which one or more programs are additionally supplied at a premium over those programs generally supplied.
CATV systems are known which provide various channel containing television programs for selective viewing by subscribers to the system. Of these various channels, one or more may be designated as premium channels for selective viewing by those subscribers who pay an additional amount for the privilege of viewing the programs on the premium channels. These programs may be in the form of first run movies, major sporting events, etc. Generally, these programs are scrambled at the cable supplier's head-end and subsequently descrambled at the paying subscriber's home in a device commonly referred to as a set-top converter.
One method of scrambling/descrambling the television programs involves suppressing the modulated TV signal during the horizontal and verticle blanking intervals making the synchronizing information contained in these intervals indiscernible to the receiving television set. Horizontal synchronization timing is then encoded by means of amplitude modulation on the audio subcarrier. An authorized descrambler then takes the horizontal synchronization timing off the audio signal, reshapes it, inserts vertical timing, and applies the resultant signal to an attenuating pad. The attenuating pad simply attenuates all of the incoming modulated TV signal except for those portions containing the horizontal and vertical synchronization information. The attenuating pad output then is a modulated TV signal with synchronization information at the proper signal amplitude with respect to the video information.
The problem then arises that unauthorized descramblers for the above system are easy to fabricate. Attempts have been made to further complicate the encoding of synchronizing information on the audio subcarrier but these attempts generally fail to achieve a much higher degree of security because frequency and phase components of the desired synchronizing information are still present. The above method also suffers from another weakness in that the encoding of the synchronizing information on the audio subcarrier is in a fixed fashion. Once the code is broken, it is broken for all time.